Please check your modem. Search the internet for AT commands that are specific to the modem you are using.
Then connect the modem(with SIM) to your serial port.
Start the Hyperterminal and then give the AT commands to send SMS.
It must work from the hyperterminal.
If it doesn't, then there is some problem with the modem and the SIM.
If sending SMS using hyperterminal works, then use this program.
It will work.

I used sms lib for my modem (handphone), it works fine, but using this script doesn't work at all.
SMSlib is too big for the size.,and this script is more simple.